USING MASS-BALANCE GEOCHEMICAL MODELING TO PARTITION THE SOURCES OF DISSOLVED IONS AT MILL SPRING IN SOUTHEAST KENTUCKY

Chemical weathering of carbonates via carbonic acid has been assumed for a long time to be the sole source of the chemical weathering of carbonates in the Interior Lowland Plateaus of the U.S.; however, it is clear from samples of spring water that other processes contribute to the flux of dissolved ions. These processes include, among others, atmospheric deposition, the potential for the hydrolysis/oxidation of sulfides in the host bedrock and entrained from underlying shallow brines, and the weathering of siliciclastic sediments transported into the karst aquifer by allogenic recharge from non-carbonate caprock. This research specifically considers quarterly water quality data from Mill Spring watershed, a karst aquifer located along the Cumberland River near Monticello, KY. This research seeks to use principal ion chemistry to quantify the magnitude of each geochemical process and assess the potential contribution of sulfur-rich brines into shallow groundwater. In this presentation, we have conducted a mass balance calculation for annual data between 2000 and 2013 using an inverse geochemical model in PHREEQC Version 2 with two end-member waters: precipitation chemistry from the National Atmospheric Deposition Program as aquifer input and annual spring data at the output of the karst aquifer. Preliminary results of molar transfers for the input-output sample pairs demonstrate that carbonic-acid speleogenesis is still the main process of carbonate dissolution (>80%), carbonate dissolution triggered by sulfuric acid derived from sulfur brines may contribute for an additional 16%, while other processes such as silicate weathering may likely contribute for the remaining 3%.